A gas turbine includes a compressor, a combustor, and a turbine. The compressor generates high-temperature and high-pressure compressed air by compressing air taken in from an air inlet port. The combustor generates high-temperature and high-pressure combustion gas by supplying fuel to the compressed air and burning them. The turbine includes a casing having a passage in which a plurality of turbine stator vanes and turbine rotor blades are alternately arranged. The combustion gas supplied to the passage is used to drive the turbine rotor blades, thereby, for example, rotatably driving a rotor connected to a generator. The combustion gas having driven the turbine is converted to static pressure by a diffuser and released to the atmosphere.
The combustor employs a premix combustion system in which a plurality of main nozzles for supplying fuel is arranged in the circumferential direction of the gas turbine. Some combustors are a cannular type in which a plurality of individual combustors is arranged in the circumferential direction of the gas turbine, and some others are an annular type that is integrally formed in a ring shape. Here, lean combustion takes place in the main nozzles by mixing air and fuel in advance. Accordingly, it is possible to suppress the combustion temperature, and reduce the generation of nitrogen oxides (NOx) due to combustion. However, in such a combustor, fuel is separately injected from the main nozzles, and under the condition when the load of the gas turbine is low such as during ignition and acceleration, the fuel flow rate per one main nozzle is low. Consequently, the fuel-air ratio (fuel flow rate/air flow rate) in the combustion region is reduced, thereby degrading the combustion characteristics. As a result, the generation of carbon monoxide (CO) and unburned hydrocarbon (UHC) is increased. To address them, the combustor includes a bypass valve, and the fuel-air ratio in the combustion region is controlled to be high, by bypassing a part of combustion air.
A conventional fuel control method is used to increase the fuel-air ratio in the combustion region, by dividing the main nozzles (main nozzle groups) into a first group and a second group, and at startup (during ignition and acceleration) and during low load operation when the fuel-air ratio is low, by supplying fuel only to the main nozzles of the first group while cutting the fuel flow to the main nozzles of the second group. Accordingly, the fuel flow rate per one main nozzle is increased (for example, see Patent document 1).
[Patent document 1] Japanese Patent Application Laid-open No. 2001-73804